Marshall Valvestate Mk1 8080 - help sought re valve access?

Voxman

I bought my 8080 brand new in 1991 when they first came out.  Had a service last year and pots were replaced.  Over the last month or so its started to howl on the gain channel and I strongly suspect that the 12AX7 has likely gone microphonic and needs changing.  I have a Russian Electro-Harmonix 12AX7 to put in which I think would be a decent replacement. 

However, the 'gubbins' are all sealed in a 'box' enclosure.  If anyone has changed a valve in these before please could you give me 'idiot proof' instructions of how to access this - even down to which screws to undo, any cabling to watch out for, any tricks to make life easier (eg presumably best to lay down amp flat?) etc etc, and where the valve is actually positioned.  I've looked on the internet but can't find any clear instructions, vids, or even pictures.

Thank you in advance guys!  

Danny1969

It's on the PCB, take out the head unit by undoing the chassis screws, slide out and then you will see the valve. Check the big resistors there while your at it 

Lebarque

I had one of them too. I clearly remember retiring it to a skip after many years of service.

Voxman

Lebarque said:

I had one of them too. I clearly remember retiring it to a skip after many years of service.

Yeeeahh...but not really too much help with my query?  

Danny1969 said:

It's on the PCB, take out the head unit by undoing the chassis screws, slide out and then you will see the valve. Check the big resistors there while your at it 

@Danny1969 - thank you for that.    What am I looking for re the resistors? (I have no electrical tools/meters to check anything)

Danny1969

There are some large ceramic resistors near the valve which get hot and sometimes crack and go open circuit if they got too hot. If they are still white with no cracks then they are fine. 

Voxman

Danny1969 said:

There are some large ceramic resistors near the valve which get hot and sometimes crack and go open circuit if they got too hot. If they are still white with no cracks then they are fine. 

Thanks Danny.  BTW, re '1969' - dob, or a guitar you have? (just curious as my hardtail Strat is 1969)

Danny1969

Voxman said:

Danny1969 said:

There are some large ceramic resistors near the valve which get hot and sometimes crack and go open circuit if they got too hot. If they are still white with no cracks then they are fine. 

Thanks Danny.  BTW, re '1969' - dob, or a guitar you have? (just curious as my hardtail Strat is 1969)

DOB mate .... I'm an old bastard

FX_Munkee

I can't remember if you have to remove the faceplate or not but, anyway be careful with the little latching switches on the front. They're very easy to get caught on their holes and break really easily. On the upside Marshall will send you replacements FOC if you phone them. Guess how I know.

Voxman

Danny1969 said:

Voxman said:

Danny1969 said:

There are some large ceramic resistors near the valve which get hot and sometimes crack and go open circuit if they got too hot. If they are still white with no cracks then they are fine. 

Thanks Danny.  BTW, re '1969' - dob, or a guitar you have? (just curious as my hardtail Strat is 1969)

DOB mate .... I'm an old bastard

Trust me Danny - you're a youngster mate!!  ..can't comment on your parentage though.  

Voxman

FX_Munkee said:

I can't remember if you have to remove the faceplate or not but, anyway be careful with the little latching switches on the front. They're very easy to get caught on their holes and break really easily. On the upside Marshall will send you replacements FOC if you phone them. Guess how I know.

Appreciate the heads up, thanks!  God forbid Marshall should have put the valve where its easy to change!  About 6 weeks ago, an internal fuse went in my DSL401 - what a palava to get to that too..that was well worth £10 for my amp tech to do it for me!  Some of these mp 'designers' need their asses kicked! 

FX_Munkee

Actually while you're in there you should modify it and add a speaker socket where the cable exits the chassis. Really easy to do.

ICBM

With the Valvestate it's extremely simple - lay the amp face down with the handle pointing towards you, undo all the screws (six if I remember correctly) in the top of the amp and the chassis will tilt back into the space behind it. You can then change the valve, which should be very obvious.

The only thing to watch out for is to make absolutely certain the amp is not connected to the mains! The valve operates at proper valve voltages so unlike most solid-state amps it's dangerous on the circuit side as well as the mains side.

Bidley

What everyone else said. It's actually pretty simple. Just be mindful of the chassis dropping when you loosen the screws (so either hold it up or lay the whole thing down, as ICBM said).

Voxman

Thanks guys - much appreciated